[otbn] Fix Verilator lint warning
[otbn] Fix Verilator lint warning

Extending the width of dmem_index_bus is no longer needed as it has the
same width as dmem_index_core now that we have increased the bus
accessible DMEM part.

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan